Properties of Nonequilibrium Materials

•Introduction

•Kinetically Disordered Intermetallic Compounds

•Superhard Materials

•Semiconductor Nanoclusters
 
 
 

Introduction

Nonequilibrium materials can be produced in many ways. In some of the more interesting nonequilibrium materials that we have produced, the challenge is to examine their properties. In other cases the challenge is in the production of the material.

 Contents

By rapid solidification we have produced chemically disordered versions of the intermetallic compounds Ni₃Al and Ni₂TiAl that, in equilibrium, are ordered all the way to the melting point [1-4]. These disordered compounds are predicted to have different properties than their ordered compounds. For example, the thermal expansion coefficients and elastic moduli of ordered and disordered Ni₃Al are predicted to be significantly different. Additionally, the lack of ductility of intermetallic compounds is blamed on the long-range order, does not permit dislocations with the smallest burgers vector to form (except as partials connected by anti-phase boundaries which inhibit their motion). The ability to compare the properties of microstructurally and chemically identical ordered and disordered materials will permit Yucong Huang and Michael Aziz to see whether this is indeed the case. If so, kinetically disordering a compound by rapid solidification (e.g., by spraying it to create a powder and then compacting the powder) may permit parts to be fabricated to net shape while still ductile. Alternatively, a laser surface treatment may disorder and soften a surface layer, which could then be removed by a cutting tool. Subsequent high-temperature annealing could bring back the long-range order and, with it, the high-temperature strength and creep resistance that makes ordered intermetallic compounds so attractive for high-temperature structural applications.
 
 

Fig. 1. X-ray diffraction scan showing kinetic disordering of intermetallic compound Ni₃Al, which in equilibrium is ordered all the way up to its melting point,  by pulsed laser melting and rapid solidification.  Superlattice peak (220) disappears after laser treatment while fundamental peak (110) remains, showing that the material has been kinetically disordered.  Long-range order is recovered with a subsequent furnace anneal.
 
 

References:

1. W.J. Boettinger and M.J. Aziz, "Theory for the Trapping of Disorder and Solute in Intermetallic Phases by Rapid Solidification", Acta Metallurgica 37, 3379-3391 (1989).

2. W.J. Boettinger, L.A. Bendersky, J.A. West, M.J. Aziz and J. Cline, "Disorder Trapping in Ni2TiAl", Materials Science and Engineering A 133, 592-595 (1991).

3. J.A. West, J.T. Manos and M.J. Aziz, "Formation of Metastable Disordered Ni3Al by Pulsed Laser-Induced Rapid Solidification", Materials Research Society Symposia Proceedings 213, 859-864 (1991).

4. J.A. West and M.J. Aziz, "Kinetic Disordering of Intermetallic Compounds by Rapid Solidification", The Metallurgical Society Symposia Proceedings, Kinetics of Ordering Transformations in Metals, edited by H. Chen and V.K. Vasudevan (TMS, Warrendale, PA, 1992), pp. 177-184.
 
 

There is currently significant experimental [1, 2] and theoretical [3, 4] interest in the synthesis and properties of carbon nitride materials due in part to the early prediction that a solid with the b - Si3N4 structure, b - C3N4, would have a hardness rivaling that of diamond [3]. Until now, the majority of experimental studies have centered on low-pressure film growth. While these studies have led to C-N materials with a wide range of compositions, including a well-defined C2N phase with some diamond-like properties [2], the local C bonding in all materials evaluated is predominantly sp2 that is typical of low-density, graphitic structures. The uniform tetrahedral sp3 C bonding expected for pure b - C3N4 or other high-density phases has not yet been achieved in low-pressure studies. Andrew Stevens, Charles Lieber, Carl Agee and Michael Aziz are exploring synthesis routes using high pressure and temperature, which was the first way that artificial man-made diamond was produced. Sufficiently high pressure should stabilize the denser sp3-bonded phases, but it is not known how high the pressure must be. Also, it is not known whether the kinetic barriers to the formation of an sp3 -bonded carbon nitride phase can be overcome at temperatures below the point that an sp2 -bonded precursor decomposes into carbon and N2.
 
 

The pressure-temperature decomposition boundaries for one particular precursor have recently been determined [1]. Above a critical temperature, decomposition proceeds extremely rapidly. Significantly, this critical temperature is rather low (in the vicinity of 600 ¡C) over the pressure range (0-20 GPa) examined. For these pressures, apparently the barrier leading to N2 formation is lower than the carbon sp2 to sp3 transformation essential for conversion to ultrahard carbon nitride. The formation of N2 is a local event with a large thermodynamic driving force and is expected to be essentially irreversible. Hence, we believe that kinetics are likely to play a dominant role in the synthesis of sp3 - bonded carbon nitrides. In theis regard it is encouraging that the critical temperature increases with increasing pressure (i.e., the barrier to N2 formation is increasing with pressure). Hence, higher pressures and temperatures may lead to a successful sp2 to sp3 transforation with the following caveats: (1) well-controlled temperatures, which are accessible through resistive or furnace heating but not through laser heating, are required to avoid precursor decomposition and (2) composition analysis of microcrystalline phases are essential to avoid misassignment of products.
 
 

References

1. see A.J. Stevens, T. Koga, C.B. Agee, M.J. Aziz, and C.M. Lieber, "Stability of Carbon Nitride Materials at High Pressure and Temperature", J. Am. Chem. Soc. 118, 10900-10901 (1996) and references therein.

2. Z. Zhang, S. Fan, J. Huang and C.M. Lieber, "Diamondlike Properties in a Single Phase Carbon Nitride Solid", Appl. Phys. Lett. 68, 2639-2641 (1996).

3. A.Y. Liu and M. Cohen, Science 245, 841 (1989).

4. D.M. Teter and R.J. Hemley, Science 271, 53 (1996).
 

Contents

There is currently a lot of excitement about semiconductor nanoclusters due in part to the prediction that small-enough clusters will act as "quantum dots" whose photoemission wavelength can be tuned by varying the cluster size. We are using two approaches to synthesize semiconductor nanoclusters. Ion implantation under controlled conditions results in the formation of nanoclusters that are, in some cases, epitaxially oriented with the matrix [1]. Pulsed laser ablation into an inert atmosphere also results in nanoclusters that can be collected and their photoluminescence properties probed [2, 3]. It has even been possible to fabricate electroluminescent devices out of thin films composed of silicon nanoclusters [4]. We are collaborating with Drs. Murakami and Yoshida [2-4] to better characterize and understand the cluster formation process. With both the ion implantation and laser ablation techniques, the current challenge is to understand the nanocluster nucleation and growth processes well enough to use them to control and sharpen the cluster size distribution.
 
 

References

1. C.W. White, J.D. Budai, J.G. Zhu, S.P. Withrow, and M.J. Aziz, "Ion Beam Synthesis and Stability of GaAs Nanoclusters in Silicon", Applied Physics Letters, 68, 2389 (1996).

2. T. Makimura, Y. Kunii, N. Ono and K. Murakami, "Visible Light Emission from SiO2 Films Synthesized by Laser Ablation", Jap. J. Appl. Phys. 35, L1703-5 (1996).

3. T. Makimura, Y. Kunii, and K. Murakami, "Light Emission from Nanometer-Sized Silicon Particles Fabricated by the Laser Ablation Method", Jap. J. Appl. Phys. 35, 4780-4 (1996).

4. T. Yoshida, Y. Yamada and T. Orii, "A Novel Electroluminescent Diode with Nanocrystalline Silicon Quantum Dots", International Electron Devices Meeting, Technical Digest, San Francisco CA 8-11 Dec 1996.

Contents

Mike Aziz